The present invention relates to a printer for printing lines of characters in which the characters are uniformly spaced in typewriters and similar office machines.
Printers of the type contemplated by the invention generally include a carriage, a carriage drive supporting the carriage for movement along a line printing path, a typing element, or type carrier, carrying type faces of the characters to be printed and movably mounted on the carriage, a typing element drive motor connected to the typing element and controllable from the keyboard for displacing the typing element in steps to bring a selected type face into a printing position, a type striking device positioned to act on the type face in the printing position to effect printing of the associated character on a record carrier medium, and a control device connected to receive signals from the keyboard, to derive therefrom driving pulses identifying each character to be printed, and to transmit those driving pulses to the typing element drive motor for bringing successive selected type faces to the printing position.
In order to obtain neatly printed copy with the characters spaced apart as uniformly as possible, it is known, in typewriters having type carriers that are driven by, for example, a summing drive, to have the type carrier cooperate with a centering device. This centering device serves the purpose of aligning the type carrier for the final printing position and of holding it in this centered position until the particular character has been printed. The final alignment of the type carrier by the centering device occurs only after the type carrier has been rotated by the summing drive into the desired orientation, dictated by the character to be printed.
Such centering device is required also because known summing drives are unable to perform the desired rotary setting of the type carrier with absolute precision since, due to the plurality of individual parts involved, there always occurs significant tolerance deviations, or play. Moreover, during the setting process, torsional vibrations occur in the type carrier setting shaft and elastic deformations occur in the setting drive, both having a great influence on the type carrier and thus making accurate setting of the type carrier impossible.
The prior art, on the other hand, also includes arrangements employing servo or stepping motor drives in which the type carrier can be moved from an initial position into a desired position in either direction of rotation. In order for the type carrier to reach the desired position as quickly as possible, it is advisable to select the shortest setting path.
These reversible drives for setting the type carrier have the drawback that the positioning accuracy exhibits a large hysteresis due to the fact that the setting may be effected from different directions. The positioning accuracy is also influenced greatly by the angle of rotation which the type carrier must traverse when it is set from an old position to a new position.
A centering device that may be provided must therefore be able to align the type carrier within this large hysteresis, which involves a certain amount of time and requires the expenditure of much force. In printers without centering devices it is impossible to produce neatly printed material with uniformly spaced characters due to the great hysteresis involved.